This invention relates to a process for the production of compacted carbon black pellets for various applications.
By virtue of their small particle size of 0.01 xcexcm to 20 xcexcm, colouring materials are known to have a strong tendency to dust and to be very difficult to dispense due to the strong adhesive forces between the particles. This is overcome by pelletising these powders before use. However, pelletisation often also results in a reduction in dispersibility as the stability of the pellets is usually increased by binders in order to improve transport characteristics. As a result, pigment pellets frequently exhibit lower initial colour intensity, such that, for a given dispersing time, the pellets develop a less intense colour than wherein colouring with powders. The desired advantages of absence of dust and good dispensability have thus resulted in considerable efforts to obtain readily dispersible pellets for pigments too.
This applies in particular to carbon blacks which, due to the small particle size and low bulk density thereof, have always previously undergone compaction as dry powders by xe2x80x9cdegassingxe2x80x9d between vacuum rollers or by xe2x80x9cbeadingxe2x80x9d in rotating drums. However, compaction between rollers does not yield pellets and dry pulverization results in agglomeration of the flocculent carbon black into large spherules of up to a few millimetres in diameter. In wet beading, the carbon blacks are pelletised in beading machines using water and possibly binders and then dried. The production of carbon black pellets is described in Ullmann""s Encyclopedia of Industrial Chemistry, fifth edition, volume A 5, page 148.
U.S. Pat. No. 4,946,505 describes the production of carbon black and pigment pellets for colouring concrete which are obtained by spray pelletisation. The disadvantage of spray pelletisation is that it is necessary to prepare an aqueous slurry of carbon black and vaporisation of the water entails considerable drying costs. U.S. Pat. No. 4,946,505 explicitly excludes compaction and briquetting processes.
DE-A 43 36 548 and DE-A 43 45 168 describe carbon black pellets which are produced with the addition of considerable quantities of water by means of an annular die press, subsequent rounding and drying. The resultant pellets contain less than 1% binder.
EP-A 0 370 490 describes carbon black flakes for printing inks which are produced by compacting carbon black under a low pressure. The material still contains dusting fractions. It is explained that dispersibility suffers if elevated pressure is used.
EP-A-0 802 241 discloses carbon black pellets which have a relative colour intensity relative to the powder on which they are based of at most 100%.
The processes hitherto available either do not yield a satisfactory material or are unfavourable in energy terms due to post-drying.
The object thus arose of providing carbon black pellets which combine mechanical stability with elevated colour intensity as well as a technically simple, low cost process which yields readily dispersible, low dusting carbon black pellets which are stable in transport.
This object is achieved by the carbon black pellets according to the invention: the carbon black powder is compacted to such an extent with the addition of auxiliary substances and subsequently pelletised that the quotient of pycnometric density and bulk density is between 3.0 and 10, preferably between 3.5 and 8. Despite the compaction, the resultant pellets surprisingly exhibit greater colour intensity than the powders on which they are based.
The present invention accordingly also provides a process for the production of carbon black pellets, in which carbon blacks are compacted with auxiliary substances, in a preferred embodiment twice or more, and commuted to yield pellets, wherein the pellets generally have an average particle size of 0.3 to 2 mm, preferably of 0.5 to 1.0 mm. The quotient of pycnometric density and bulk density of the pellets obtained according to the invention is between 3.0 and 10, preferably between 3.5 and 8.